为什么此C程序的输出不正确

Question

I have written ac program like this and expect to get correct outputs.

 #include <stdio.h>
 #include <stdlib.h>

 int main()
 {
     int t;
     scanf("%d", &t);
     int a[t-1], b[t-1];
     //printf("%d\n",sizeof(a) );
     do{
     scanf("%d %d", &a[t-1], &b[t-1]);
     }while(--t);

    do{
         printf("%d\n",a[t-1] + b[t-1]);
    }while(--t);

    return 0;
 }

But I am getting random values.What is wrong with this program? The attempt is made to enter number of tries, then enter values , then print the addition of those values.

PS the array initialization is dynamic here.Is it valid?If not what should be the correct way?

Answer 1

When you declare an array of length 10 , the index is from 0 to 9 . There is no index 10 for that array.

In your code, your array is of size t-1

int a[t-1], b[t-1];

So, in your first iteration, the scanf() and printf() statement accesses a[t-1] , which is not present. And, since its a do while loop, it is guaranteed to run atleast once.

Accessing array outside its bounds is Undefined behavior . So, as Cool Guy pointed out in the comments, anything can happen.

Answer 2

Between reading all values and printing all values, you never reset t to it's original value. You will need to do that, probably using another variable to save it's original value.

Answer 3

The array size should be a constant, it cant be set at run time. For run time memory allocation you need to use pointer variable with malloc function. So first of all you need to assign a constant value to variable t at time of declaration.

Just a tip, always try to declare and initialize the variable before any statements. The array initialization should come before scanf, by doing this you can clearly see the problem ie you are allocating memory(some random) and then getting value from the user. The compiler must be doing the similar thing, allocating some random size memory and then getting the value t, now this value t will only be used in your iterations and not for array allocation as arrays are allocated before with some rand size (what ever garbage value t is pointing to at time of its declaration). Sometime the program will be displaying some random values and sometime it could crash on accessing a memory that's out of bound.